The cyclopolymerization of IPDI is already known. According to German Published Patent Application No. 1,934,763, a reaction mixture which contains polyisocyanates with isocyanurate groups in addition to the dimerization product is formed when phosphines are used as catalysts.
British Pat. Nos. 1,391,066 and 1,386,399 describe the polymerization of IPDI using alkali metal phenolates as catalysts. The use of such catalysts has the advantage over catalysis with phosphines that the reaction products consist almost entirely of trimers and virtually no dimerized polyisocyanates are formed. However, the use of such catalysts had serious disadvantages, as explained below.
The polymerization reaction is very vigorous and it is presumably for this reason that in all of the examples given, the reaction is carried out in the presence of a solvent.
Separation of unreacted free IPDI proves to be extremely difficult because the polymerization products are not uniform and have a high viscosity. Furthermore, the catalyst must be neutralized to terminate the reaction, and this causes cloudiness due to the resulting salts. The use of the products of British Pat. No. 1,386,399 in the production of lacquers is consequently very difficult if not impossible.
There have therefore been many attempts to improve further the desired polymerization of IPDI. German Published Patent Application No. 2,325,826 and its equivalent U.S. Pat. No. 3,919,218 discloses a process for the preparation of stable isocyanato-isocyanurate solutions in which IPDI is used, inter alia, as a starting material. In this process, aziridine or an aziridine derivative in combination with a tertiary amine is used as the catalyst. This process has the disadvantages that aziridine is dangerous to handle because of its well-known carcinogenic properties and that a part of the catalyst must be removed by distillation. Moreover, polymerization carried out by this process has an incubation period of several hours, during which time the diisocyanate must be heated. When the exothermic reaction finally begins, the temperature rises sharply within a few minutes. A solvent must be added to the reaction mixture to prevent excessive turbulence. Moreover, since this process takes several hours, it is considered time consuming.
A commercially viable, simple process for the production of polyisocyanates containing isocyanurate groups from IPDI is unavailable, for the reasons given above. Since, a substantially colorless polyisocyanate containing isocyanurate groups based on IPDI and free from solvents and substantially free from monomers would be an extremely interesting polyisocyanate for lacquers, it was an object of the present invention to provide such a process.
This problem could surprisingly be solved by using quite specific quaternary ammonium hydroxides as catalysts for the trimerization of IPDI.
The use of quaternary ammonium hydroxides as trimerization catalysts for isocyanates has already been disclosed in German Pat. No. 1,150,080, but the ammonium hydroxides disclosed in the said publication are hardly suitable as commercial catalysts, in particular for the solvent-free trimerization of IPDI, because the reaction is not controllable in the absence of solvents. When attempts are made to trimerize aliphatic or cycloaliphatic diisocyanates with the ammonium hydroxides mentioned in the aforesaid publication, a sudden vigorous exothermic reaction is frequently observed after a substantial incubation time. The examples given in German Pat. No. 1,150,080, therefore, mainly describe only the trimerization of aromatic isocyanates. The only example of the trimerization of an aliphatic isocyanate is the trimerization of hexadecyl isocyanate mentioned in Example 13. After a reaction time of 4 days, the corresponding trimer is obtained in a yield of less than 50%. The catalysts given in German Pat. No. 1,150,080 are unsuitable for economical solvent-free trimerization of IPDI by a reproducible and easily controlled reaction.
The quaternary ammonium salts, ammonium alcoholates and ammonium phenolates mentioned in German Published Patent Application 2,631,733 (equivalent U.S. Pat. No. 4,040,992) and in British Pat. No. 1,465,812 are also unsuitable as catalysts for such a reaction because the compounds are not easily destroyed by heat and must therefore be inactivated by the addition of a catalyst poison when the desired degree of trimerization has been reached which again leads to undesirable impurities in the end product.
The catalysts to be used according to the invention, on the other hand, make it possible for polyisocyanates containing isocyanurate groups to be obtained from IPDI without the use of solvents, and inactivation of the catalyst, for example by the addition of a catalyst poison, is unnecessary since the compounds used according to the invention as catalysts are decomposed by heat during the reaction, thereby losing their activity.